Whats the rule of thumb when adding HP? For example, add 100HP per engine, same prop, how many RPM should one gain roughly??

Ther are lots of variables, but on an average V-hull running 60 to 70 you should pick up four or five hundred RPM and six or seven mph.

oops, just noticed you said per engine. In that case double it.

I was going to guess.....and it is a guess, that your original info is correct. Editing your post to "double it" I think is wrong.

No need to guess, it's actually fairly simple math. There are some variables, but adding 200hp to even a slowish v-hull running in the mid 60's will net you about 10mph or 700 RPM at 12% slip. A more efficient hull will see bigger gains..

pitch x rpm / (gear ratio x 1056) = Theoretical speed without slip.

http://www.go-fast.com/boat_speed_predictions.htm
http://www.go-fast.com/Prop_Slip_Calculator.htm

Maybe you can figure out something using these.

I added about 60hp to a 24 Outlaw and gained about 400rpms with the same prop. This was on a 60mph boat that went to about 65mph.

It is going to depend a lot on where you are starting. 60hp added to a 700hp engine in a 90 mph boat will not net the same results.

Correct. Lots of variables at play.

Post your current wide open throttle RPM, GPS Speed, prop pitch, and drive gear ratios. We can figure out your prop slip (aka efficiency) and guestimate what 100 more horsepower will push your boat. (or 100hp per engine if it's a twin)